Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $20.77   5.356%   $21.8824   $21.88   $21.90   $21.90  
  Purchase #2 $90.32   13.203%   $102.2449   $102.24   $102.25   $102.20  
  Purchase #3 $188.87   4.760%   $197.8602   $197.86   $197.85   $197.90  
  Purchase #4 $144.51   9.247%   $157.8728   $157.87   $157.85   $157.90  
  Purchase #5 $69.70   13.205%   $78.9039   $78.90   $78.90   $78.90  
  Purchase #6 $124.64   3.791%   $129.3651   $129.37   $129.35   $129.40  
  Purchase #7 $115.65   10.122%   $127.3561   $127.36   $127.35   $127.40  
  7 purchase totals = $754.46       $815.4854   $815.48   $815.45   $815.60